A photovoltaic system for generating power using sunlight includes a solar array connected to the utility grid or a load through a power conditioner including components such as an inverter, and feeds power generated in the solar array to the utility grid and the load.
Recent photovoltaic systems have higher voltages, and increasingly use inverters including no transformers to achieve higher efficiency. This may cause a large potential difference to appear between the solar cells and the grounded frame. The potential difference, combined with external factors such as temperature and humidity (high temperature and high humidity), is known to produce a leakage current, which can possibly cause the potential induced degradation (PID) phenomenon.
FIG. 47 is a conceptual illustration of an example PID phenomenon. FIG. 47 shows one solar module string 10 included in a solar array in a photovoltaic system. The solar module string 10 includes multiple solar modules (solar panels) 1 that are connected in series. The solar module string 10 is connected to a utility grid 40 through a power conditioner 30. Each solar module 1 in the solar module string 10 receives sunlight during the daytime to generate power, thus producing a potential difference between a positive input terminal 301 and a negative input terminal 302.
FIG. 48 is a schematic diagram showing the structure of the solar module 1. As shown in FIG. 48, the solar module 1 includes a frame 11, a back-sheet 12, a cell 13, a glass sheet 14, and a sealant 15.
The cell 13 is a device including a semiconductor layer (power generation layer) that converts light energy into power with the photovoltaic effect. The glass sheet 14 is placed above the cell 13 on its light-receiving side, whereas the back-sheet 12 is placed below the cell 13 on the light non-receiving side. The cell 13 is sealed by the sealant 15 placed between the glass sheet 14 and the cell 13 and between the cell 13 and the back-sheet 12. The frame 11, which is on the outer periphery of the solar module 1, fixes the solar module 1 onto a support. The frame 11 is formed from conductive metal, and is grounded.
Of the solar modules 1 that are connected in series, as shown in FIG. 47, solar modules 1 adjacent to the input terminal 301 include cells 13 having positive potentials to ground, whereas solar modules 1 adjacent to the input terminal 302 include cells 13 having negative potentials to ground. As the difference between the potentials to ground increases, a leakage current may appear between the cell 13 and the frame 11 in a solar module 1 or between water 91 on the glass surface and the cell 13 as indicated with dashed arrows in FIG. 48. In this case, sodium ions in the glass sheet 14 and other components may move to the cell 13 and interfere with the electron transfer of the cell 13 to reduce the performance of the cell 13 or to cause the PID phenomenon. For example, in a cell of the solar module 1 including a p-type semiconductor, a negative potential to ground can reduce performance. In a cell of the solar module 1 including an n-type semiconductor, a positive potential to ground can reduce performance.